The science and magic of jam-making

Jam-making conjures up images of domestic idylls, an escape to the mountains to live on your wits, and jam. The seemingly simple mixture of fruit and sugar held together by a web of pectin strands can be both beautiful and maddening. A jam worthy of a Women's Institute rosette, however, might have a nature so tender that it quivers when cut with a spoon to reveal sparkling, ruby-like faces.

This sweet treat was named late, in the early 1700s, but "jam" captures the difference between it and the (in my opinion) inferior jelly. Jellies are made from homogeneous fruit juice with none of the wonderful texture-giving "crushed" fruit.

Jam as we know it only seems to have emerged in the 19th century. It took a cheap and reliable source of sugar from the West Indies to make jams affordable. Before this, sugar was considered a spice and the price in Europe was such that only the richest could afford it. Preserves made from sugar were too precious to spread thickly on toast. Instead they were eaten as "spoon sweets" with feasts being capped off with the distribution of delicate silver spoons laden with fruit preserves. You may still be offered such treats with a glass of cooling water in the Middle East and eastern Europe.

The first European sugar preserves made use of that seemingly magical substance, honey. The earliest fruit preserves would be made by mixing fruit pulp with honey and allowing it to dry in the sun, creating a texture more like that of a jellied sweet.

The high-pectin quince lent itself to making this well-set fruit preserve. In Greece, a common spiced preserve of quinces was known as melomeli (apple honey) and was thought to be an aphrodisiac and to aid digestion. In Britain, it was adapted to incorporate other fruits, such as pears, damsons, plums, and finally Seville oranges, becoming marmalade. Eventually, when sugar prices fell late in the 17th century, marmalade became a soft jelly that, smeared on toast, became a staple of the Scottish breakfast.

Jam only reached the masses in the 1880s when it was used to enliven the dark wholemeal bread eaten by the working classes. Many of these factory-produced jams contained more sugar and colour than fruit. The quality of commercial jams have improved greatly since then but they are still the sickly sweet sisters of a good homemade jam. And besides, buying jam gives no way near the satisfaction of making it yourself.

The recipe

Jam recipes mostly comprise equal weights of fruit and sugar. You can play with this 1:1 ratio as much as you want, but too much fruit and you may lose the preserving effects of the sugar; too much sugar and it may crystallise during storage.

The choice of fruit for jam-making is almost endless. I always try to use seasonal fruit to get the best flavour for my jam. Slightly unripe or "just ripe" fruit will form a jam more easily than very ripe fruit as it contains more pectin and is more acidic.

1kg fruit
1kg granulated sugar
Lemon juice and/or pectin (depending on the fruit you use)

Start by removing any leaves and twigs, wash the fruit if you feel it necessary, and remove any stones. Add the fruit to a pan big enough to ensure the fruit does not reach more than halfway up the side.

Heating

Place your pan on a low heat. As the fruit heats through, a glorious fresh, warm smell will fill the air. Prolong this by heating slowly until a very gentle boil is reached. Cook until tender - any longer and the fruit will lose its shape. No sugar is added at this stage because a high sugar concentration can cause water to be removed through osmosis and result in hard, unappetising fruit. You might need to add a little water though if your fruit is very dry.

Boiling is key to jam-making because it releases a long fibrous compound known as pectin. Even though pectin only makes up 0.5-1% of the jam, you will have to learn to play it like a snake charmer or you will add your tears to your mixture.

The first handling of a jam the morning after making is full of trepidation. The jam maker's nightmare is to find a wet, sloppy strawberry sauce, not the semi-rigid, elastic substance that chemists describe as a "gel": a liquid dispersed in a solid. Pectin forms the solid that holds the liquid together. Some fruits, including apples, blackberries and grapes, can do this alone as they contain sufficient pectin. Some fruits are low in pectin, however, and so need a little more help, for example apricots, rhubarb and strawberries.

You can add commercial pectin, which is extracted from the white inner skin (the pith or "albedo") of citrus fruits or from apples. You can also buy special jam sugars with added pectin. But jam makers of yore discovered through trial and error that if they mixed low-pectin fruits with high-pectin fruit (often apple) they could create the perfect consistency. Personally, I like to mix high and low pectin fruits to keep it "in the garden", for example I might add a cooking apple to my blackberry jam.

Pectin was first isolated by French chemist Henri Braconnot in 1825 and was named from the Greek pektikos, which means congealed or curdled. It is a polysaccharide so, like cellulose and starch, it is made up of long chains of sugar molecules. In fruit, pectin is concentrated in the skins and cores where it acts as structural "cement" in the plant cell walls. In jam, pectin forms a mesh that traps the sugary liquid and cradles suspended pieces of fruit.

Branches that stick out from the long chains of pectin bond with each other to form the three dimensional network that jam makers crave. In a solution, these branches are reluctant to bond, first because they attract water molecules, which stops them bonding, and second because they have a slight negative electrical charge, which means they repel one another.

To solve the first problem we add sugar, which binds to the water molecules and frees up the pectin chains to form their network. The negative charges are reduced by acid naturally found in the fruit or added to the mixture. The acid reduces the electrical charge on the pectin branches and so allows them to bond. To increase acidity lemon juice can be added. But be careful: if your mixture is too acidic, this will damage the pectin.

As a rough guide, the juice of a whole lemon (30-40ml) will be needed for very low acid fruit, whereas half a lemon will be enough for medium acid fruit, and you won't need any for the high acid fruits. In general, fruit with high pectin will also have high acidity and vice versa.

Adding the sugar

Add the sugar and stand back as it starts to foam up the sides of the pan. A sentimentality-inducing childhood smell of sweet fruit fills the air. Allow the sugar to dissolve over a low heat then bring rapidly to the boil. Avoid stirring at this point as you may break up the fruit or cause crystallisation. A foamy scum may form on the surface of the jam; this is normal and can be removed by adding a little butter (about 20g) to break the surface tension or by skimming it off with a spoon while your mixture is cooling.

This is the exciting bit: the smell of jam fills the air and you're desperate to get it into jars and on to some toast, but patience is required. However, you will normally have to wait around 5-20 minutes for the pectin network to form. The time varies depending on the type of fruit, the type of pan etc. A wide-mouthed pan is ideal as it allows water to escape, helping to bring our precious pectin molecules closer together.

Time to pour

There are many ways of telling when your pectin network has formed and you are ready to pour the jam out. It normally forms at around 104-105C, when the sugar content is high enough to allow the pectin branches to join. Unfortunately, temperature is not a reliable signal because it varies according to acidity, amount of pectin, etc. My preferred method is direct measurement. Pour a little blob of jam on to a cooled saucer, let the jam cool in the fridge and then push against the side of it with your finger. If the surface wrinkles it means the pectin network has solidified, setting point has been reached, and you should take the mixture off the heat. If you don't boil it long enough the pectin network will not form properly. Boil it too long you risk not only losing the fresh flavour and colour of the jam but having a jam with the texture of set honey.

Cooling and decanting into jars

This is my favourite part, but I allow the jam to cool and thicken for about 10 minutes before pouring it into jars, to prevent the fruit from floating to the top. Try not to leave the jam too long, however, as lukewarm jam is a great breeding ground for mildew spores which are present in the air.

To keep you busy while you are waiting, get your pre-sterilised jars ready. You will need five or six of them. My preferred method of sterilisation is to wash them in soap and hot water, rinse them with clean water to remove any detergent, and dry them in the oven at about 160C.

Jams can remind us of summers past, even summers several years gone. It is the sugar and acid that makes this possible. Jams usually contain about 60% sugar, which is enough to stop most microorganisms growing. The high acidity also makes it an unpleasant place to breed. However, some moulds can grow even in these harsh conditions and so it is important to take care when preparing and sterilising your jars.

The satisfying gurgle of jam being poured is music to the ears. Each jar should be topped up to just less than a centimetre below the surface.

Capping and storage

I remember being puzzled why my parents always put a waxed paper disc on the surface of their homemade jam. I now know that it prevents the condensation of water on the jam's surface. Condensed water would dissolve sugar, producing an area of low sugar concentration and allowing mould growth. I must confess that jam never sits in my cupboard long enough to worry about this.

Now that our jam-making is at an end, there is only one stage to go: eating. I always struggle not to get overexcited and try my jam straight away before it has developed its "quiver". I am torn; I feel I should wait until the autumn, when I can close my eyes and relive summer. But I'm realistic. I wait till the next day to spread the noble jam thickly on a delicious chunk of simple bread and butter.

Photo: Getting the pectin content of your jam right makes the difference between a semi-rigid, elastic 'gel' and sloppy fruit sauce. Photograph: R White/Corbis